1. Field of the Invention
The present invention relates to a coordinate input device for inputting coordinates to a screen, and a display device including the same, and more particularly, to a technology suitable for achieving high coordinate detection accuracy in a coordinate input device of a capacitive coupling system.
2. Description of the Related Art
An image display device that has a sensor function in a display screen and a screen input function of inputting information by a touching operation using a finger or a stylus pen is used for a mobile electronic device such as a PDA or a portable terminal, various home electric appliances, or a stationary customer guidance terminal such as an automatic reception machine. As a sensing system used for the image display device which has such a screen input function, there are known a system for detecting changes in resistance value of a touched portion, a system for detecting changes in capacitance thereof, a system for detecting changes in light intensity of a portion shielded by touching, and the like. Among these systems, the system for detecting changes in capacitance has particularly grown in usage because of little influence on how a displayed image is seen and high durability.
U.S. Pat. No. 6,466,036 discloses a system for detecting changes in capacitance. This system is referred to as an electric charge transfer system. When a human finger or the like touches a sensor, a capacitance is generated between the finger and an electrode disposed in the sensor. Detection means includes a current source, an electric charge storage capacitor, and electric charge detection means. The detection means charges the capacitance, and transfers electric charges charged in the capacitance to the electric charge storage capacitor to charge the electric charges, and detects the amount of charged electric charges. When there is no touching on the sensor by a finger or the like, no capacitance is generated, and hence electric charges to be charged in the electric charge storage capacitor are reduced. Thus, the disclosed system is a system which determines presence of touching by a finger or the like based on the amount of electric charges charged in the electric charge storage capacitor.
U.S. Pat. No. 7,312,616 discloses a capacitance detection system based on a different system. This system is referred to as a successive approximate capacitance system. When a human finger or the like touches a sensor, a capacitance is generated between the finger and an electrode disposed in the sensor. Detection means includes a current source, an electric charge storage capacitor, and electric charge detection means. First, the detection means repeats charging and discharging of the capacitance at a fixed cycle, thereby taking out approximate constant currents from the current source and the electric charge storage capacitor. As a result, a charging potential of the electric charge storage capacitor charged to a fixed potential beforehand is lowered, causing a change in charging period of time required until the electric charge storage capacitor is charged to the fixed potential again. This temporal change depends on the amount of electric charges taken out by the charging and discharging. Further, the amount of electric charges taken out by the charging and discharging depends on a value of a capacitance generated between the electrode in the sensor and the finger or the like. Thus, measuring a change in charging period of time enables determination of presence of touching on the sensor by the finger or the like.
U.S. Pat. No. 7,235,983 discloses a capacitance detection circuit in a capacitive coupling system. In a detection system of the disclosed capacitance detection circuit, electric charges are charged in a sensor electrode capacitor for detecting a capacitance at a fixed voltage, and the amount of electric charges is detected by an integration circuit at a subsequent stage. In order to cancel offset of an operational amplifier used in the integration circuit, an integration voltage at the time of charging and an integration voltage at the time of discharging from the sensor electrode capacitor are alternately measured by switching control.
However, when the sensor employing the capacitance detection method disclosed in U.S. Pat. No. 6,466,036 or U.S. Pat. No. 7,312,616 is installed close to a display surface of a display device such as a liquid crystal display, influence of radiated noise generated by an operation of the display device may cause a problem such as a reduction in measuring accuracy. Thus, there is a need to establish a system which reduces influence of radiated noise or the like in the discussed usage form.
Referring to FIG. 43 and FIGS. 44A and 44B, the problem with the capacitance detection system disclosed in U.S. Pat. No. 7,235,983 is described. FIG. 43 simplifies a conventional capacitance detection circuit DCKT. The conventional detection circuit DCKT is constituted by an integration circuit which includes a switch SA for charging a capacitance detection sensor electrode (capacitance) Cs with a fixed voltage VDD, a switch SB for transferring electric charges charged in the sensor electrode capacitance Cs to the integration circuit, a switch SR for resetting an integration capacitor Ci and an integration value, and an operational amplifier. A terminal vi is an input terminal which interconnects the detection circuit DCKT and the sensor electrode capacitance Cs, and a terminal vo is an output voltage terminal of the detection circuit DCKT.
Each of FIGS. 44A and 44B is a waveform diagram when an integration voltage obtained by one charging or discharging operation for the sensor electrode capacitance Cs is set as an output signal of capacitance detection. FIG. 44A is a waveform diagram when only the sensor electrode capacitance Cs is connected to the input terminal vi in FIG. 43. FIG. 44B is a waveform diagram when touching occurs on the sensor electrode capacitance Cs to add a new input capacitance Cf. At a detection cycle Tdec_r, a period ta_r is a resetting period of the integration capacitor Ci, in which the switch SR is turned ON to reset an integration voltage charged in the integration capacitor Ci. Then, in a period tb_r, only the switch SA is turned ON to charge the sensor electrode capacitance Cs and the input capacitance Cf with the fixed voltage VDD. Lastly, in a period tc_r, the switch SA is turned OFF while the switch SB is turned ON to transfer electric charges charged in the sensor electrode capacitance Cs and the input capacitance Cf to the integration circuit. In the case of only the sensor electrode capacitance Cs as illustrated in FIG. 44A, an integration voltage becomes −VDD·Cs/Ci. In the case of the input capacitance Cf as illustrated in FIG. 44B, an integration voltage becomes −VDD·(Cs+Cf)/Ci. In the coordinate input device, these integration voltage values (output signals) are digitized by AD conversion, and then a difference between a digital signal when there is no input and a digital signal when there is an input is detected as an input signal to calculate touch coordinates.
In this case, though depending on a size of an input screen, the sensor electrode capacitance Cs is as large as several tens of pF, while the input capacitance Cf by a capacitance generated by a finger or the like is as small as several pF or smaller. Thus, even if an output voltage of the integration circuit is amplified by an amplifier before AD conversion, sufficient amplification is impossible due to offset of the integration value by the sensor electrode capacitance Cs, causing a problem that a dynamic range of the input signal after the AD conversion is narrow and input coordinates may not be accurately detected.